Roof Garden Over a Post-Tensioned Podium in Makati: Loading, Waterproofing, and Contractor Substitutions
A 2,400 m² roof garden is proposed on the eighth-level deck of a two-tower residential podium in Makati, 26 m above street grade. The post-tensioned two-way slab was designed for 4.8 kPa of superimposed dead load for finishes and planters. The landscape concept shows 300 mm of planting soil across the whole deck, a 200 mm gravel drainage layer, a 400 mm-deep reflecting pool, and eight mature trees in 1.2 m-deep raised planters. The contractor has submitted substitutions to save PHP 3.2 million: delete the drainage layer and rely on four roof drains; use the single-ply waterproofing membrane itself as the root barrier; irrigate with potable water through Schedule 40 PVC; and omit movement joints because 'the deck is one pour.' The developer wants occupancy certificates in five months and asks you to approve the substitutions so the schedule holds.
1. Check the proposed assembly against the 4.8 kPa structural allowance. What has to change before the design can proceed?
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The assembly fails the structural check by roughly a factor of two, and the failure is not marginal.
Proposed loads (saturated):
- 300 mm soil at about 18 kN/m³ = 5.4 kPa
- 200 mm gravel at about 20 kN/m³ = 4.0 kPa
- Reflecting pool, 400 mm water at 9.81 kN/m³ = 3.9 kPa over its footprint
- Total soil-plus-drainage alone = 9.4 kPa against a 4.8 kPa allowance
The trees in 1.2 m planters are additional, and their weight is concentrated rather than uniform.
What must change:
- Replace the soil with an engineered lightweight medium (mineral fraction plus compost, saturated unit weight around 11–12 kN/m³) at 200 mm, giving about 2.3 kPa.
- Replace the 200 mm gravel with a 25–40 mm drainage composite with filter geotextile, adding a few tenths of a kPa.
- Delete the 400 mm pool, or reduce it to a shallow 100–150 mm water feature located over a beam and column line, with its reservoir coordinated with the structural engineer.
- Reduce tree planters to a smaller number and position them over columns, with a structural check on each.
At that point the build-up is roughly 3.0 kPa, leaving margin for pavers and maintenance loads. The structural engineer must confirm in writing before any shop drawing is issued; the landscape architect cannot approve an overload.
2. Evaluate each of the contractor's four substitutions. Which would you accept, reject, or accept with conditions, and why?
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Drainage layer - reject. Four roof drains with no drainage path means water sits in the soil profile, keeping the medium saturated (which is exactly the load case above) and drowning roots. Require a drainage composite with filter geotextile, 1–2% falls to drains, overflow scuppers, and a leak-detection layer.
Membrane as root barrier - reject. A single-ply waterproofing membrane is not a root barrier; roots exploit seams and laps. Require a dedicated HDPE root barrier, continuous, turned up at all edges and penetrations, with laps sealed per manufacturer's detail.
Irrigation - accept with conditions. Potable water is wasteful and unnecessary; require stored rainwater or condensate recovery, drip with pressure-compensating emitters, a backflow preventer, zoning and a controller with a rain sensor. PVC is acceptable for buried mains but must be protected from UV and from settlement at slab joints.
Omit movement joints - reject. The waterproofing and the paving must have movement joints aligned with the structural joints and the parapet. Omitting them converts normal slab movement into membrane rupture.
Before planting, require a flood test of the membrane and a witnessed inspection of the root barrier and drainage layer.
3. Describe the layered build-up from structural slab to finished surface, plus the quality-control steps you would insist on before planting.
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Build-up, slab upward:
- Structural slab, cleaned; movement joints located.
- Primer and waterproofing membrane, with a flood test and photographic record at every penetration.
- Dedicated HDPE root barrier, turned up 150 mm at parapets and upstands, laps sealed.
- Protection layer (geotextile or drainage composite with bonding membrane).
- Drainage composite with filter geotextile, falls 1–2% to drains, plus a leak-detection grid.
- Engineered lightweight planting medium at 200 mm, or the specified depth in tree zones.
- Surface: mulch, or pedestal-set pavers on adjustable supports over the membrane zone.
Quality control before planting:
- Flood test witnessed by the landscape architect and the waterproofing applicator, with the warranty in hand.
- Inspection of root barrier continuity, especially at drains and upstands.
- Drain flow test; confirm overflow scuppers are clear.
- Medium tested for density, pH, organic content, and permeability against specification.
- Irrigation commissioning: pressure, coverage, backflow test.
- Handover of an as-built drawing showing every penetration and a maintenance manual for the owner, since access for repair on a podium is expensive and disruptive.